In Parkinson’s disease, nerve cells in the brain that produce the chemical dopamine begin to die. Lower dopamine production can lead to tremors, balance problems, stiff facial expressions and muffled speech, among other problems.

An estimated one million people in the United States have the disease, and another 60,000 receive the diagnosis each year. Although the condition usually develops after the age of 60, 15 percent of those who learn of the diagnosis are under 50.

In the latest research, 195 people with movement and balance problems caused by Parkinson’s disease were recruited from four Oregon cities. The patients were divided into three exercise classes that met for an hour a day, twice a week. One group took part in an extensive stretching class, another was taught resistance training, and the third group performed tai chi.

After six months, patients in the tai chi group performed better on a number of measures related to strength, movement control, balance, stride length and reach. Resistance training also offered some benefits, and both the tai chi and resistance training groups had fewer falls than the stretching group.

The findings are good news for people with Parkinson’s, who often lose the ability to maintain standing balance and have difficulty walking and are at risk for frequent falls. Although some symptoms of Parkinson’s, like tremors, can be relieved by drug therapy, balance and walking problems are typically not helped by drug treatment.

“Current medications do not work well in terms of providing relief for impaired balance or postural instability,” said Fuzhong Li, a research scientist at Oregon Research Institute in Eugene and the study’s lead author. “In fact, patients suffer a great deal of medication side effects which may lead to further deterioration in balance control.”

Why tai chi helps people with Parkinson’s isn’t entirely clear. Even for a healthy person it can be hard to maintain balance and stability while performing the continuous, rhythmical, weight-shifting movements of tai chi. It may be that practicing tai chi trains the parts of the brain that control balance and movement to adapt more quickly in response to the motor control challenges of Parkinson’s.